Conventional Raman spectrometers use a fixed wavelength laser source and a notch filter on the return path to block the source wavelength but allow the Stokes-shifted wavelengths to pass. A dispersive element is used to then to spatially disperse the Raman response. A multi-element detector is used to detect the dispersed Raman spectra. The size of detector is selected so that it provides enough elements (pixels) such that each pixel provides one wavelength resolution element.
In these conventional systems, resolution is generally limited by the entrance slit as it is imaged onto the detector and thus convolves the spectrum. The physical size of the detector's active area and the spatial dispersion (product of angular dispersion and lens focal length) set the range of spectrometer wavelength coverage. Decreasing the pixel size increases spectral resolution up to the limit imposed by the entrance slit. Alternatively for fixed pixel size, resolution may be increased by increasing detector size and spatial dispersion. Unfortunately, with increases in size, the detector array becomes more costly, especially for non-silicon detectors. The optics also becomes more difficult to design and more costly due to the increased field requirement at a fixed numerical aperture.
There are three principal problems with conventional Raman spectrometers.
The first is the size of the detector, grating, and optics. This leads to an instrument that is physically large—typically too large for a handheld configuration.
The second problem, for spectrometers that use multiple detector elements to increase speed of measurement, is the cost of the detector array. A non-silicon detector with many pixels is expensive. Since many materials fluoresce when illuminated with source wavelengths in the visible or short wavelength infrared (IR), it is preferable to excite Raman samples with a near infrared source. This pushes the Raman, Stokes-shifted spectrum into the near IR (NIR) as well, forcing the use of detector materials such as InGaAs. The cost of InGaAs arrays is high, with the driving cost parameter being the area of InGaAs in the array. Thus anything that can be done to reduce the length of InGaAs required to detect the spectrum can provide significant cost advantages.
Finally, the third problem is that for non-silicon based detector arrays the smallest commercially available pixel size is often limited. In the case of InGaAs, the smallest available pixel pitch is currently limited to 25 micrometers (μm) and thus forces long arrays for high resolution.
Recently, an alternate approach has been proposed in U.S. Pat. Appl. Publ. No. 2005/0264808 A1, which is incorporated herein by this reference, and demonstrated. It uses a multi-order filter, such as an etalon of finesse N, which only allows certain discrete narrow bands of wavelengths (of a periodic nature) to pass. By tuning the excitation laser or the etalon, the Raman spectrum is swept through these N pass bands (filter orders) and detected on the multi-element detector. This technique eliminates the entrance slit and pixel limitations on resolution provided that the orders are well separated on the detector. Full separation limits how small the detector area can be for a given number of orders, entrance slit width, and multi-order filter characteristic.
The multi-order filter Raman system provides the multiplex advantage while simultaneously providing a fine spectral resolution governed by the multi-order filter's transmission characteristics. The spacing required between orders to keep crosstalk to an acceptable level dictates the length of the InGaAs array for any fixed number of orders N (fixed multiplex advantage).